The present invention relates to a method for producing an electrophotographic recording material of the type composed of selenium, selenium alloys or selenium compounds applied to a conductive carrier.
Electrophotographic methods and apparatus are widely used in the reproduction art. They utilize the property of such photoconductive material whereby its electrical resistance changes when it is exposed to a suitable activating radiation.
By electrically charging a photoconductive layer and then exposing it to such activating radiation in a pattern determined by an optical image, it is possible to produce thereon a latent electrical charge image which correponds to the optical image. At the exposed points there occurs such an increase in the conductivity of the photoconductive layer that a part or substantially all of the electrical charge can flow off through the conductive carrier while at the unexposed points the electrical charge remains substantially unchanged. More precisely, the quantity of charge flowing off is greater at the exposed points than at the unexposed points. The latent image can be made visible with a picture powder, a so-called toner, and the resulting toner image can finally be transferred to paper or some other medium, if this should be required.
Organic as well as inorganic substances are used as the electrophotographically active substances. Among them, selenium, selenium alloys and selenium compounds have gained particular importance.
In order to produce pictures having good contrast and sharpness, it is necessary, inter alia, for the charge applied to the photoconductive layers to flow out from the exposed parts after exposure uniformly and without interference. For this, an intimate and uniform contact between the conductive layer substrate and the photoconductive layer is required. Since the layer substrate and the photoconductive layer are composed of substances having very different coefficients of thermal expansion, the requirement for intimate and uniform contact can be met only with difficulty in the known embodiments of electrophotographic recording materials. This applied particularly to arsenic-containing selenium alloys having more than 20 percent by weight arsenic, such as As.sub.2 Se.sub.3, which is applied, for example, to a glass substrate with a conductive intermediate layer of tin dioxide. Such layers have only poor adhesion which adversely affects their application.
If the photoconductive layer is applied by vapor deposition at a high substrate temperature, particularly at a temperature above the glass transformation temperature, the substrate is initially well wetted by the photoconductive layer. It should be noted that the "transformation temperature" of glass is defined as being that temperature at which glass has a viscosity of 10.sup.13.4 poises. However, when the substrate is cooled to room temperature, stress cracking resulting from the different coefficients of expansion of the layer and the substrate causes the photoconductive layer to come loose from the substrate.
If, on the other hand, the vapor-deposition is effected at low temperatures in order to avoid stresses during cooling, the wetting of the substrate by the photoconductive layer is so poor that the photoconductive layer will also soon come loose from the substrate.